A method for controlling a cooktop including a cooking zone which can be heated by an induction heating device is described in German Patent DE 10 2004 002 058 B3. The described method controls a cooking process on a cooktop including a cooktop panel which is made, in particular, of glass-ceramic and which has a material thickness s defined by a flat upper surface and a flat lower surface in a direction perpendicular to the main directions of extension of the cooktop panel, further including at least one cooking zone that can be heated by a heating device located beneath the cooktop panel when the cooktop is in the installed position, further including an electrical control system for controlling the heat output of the heating device, and further including heat sensor units located beneath the cooktop panel.
In order to enable the heat output of the heating device to be controlled taking into account the influence of a cooking utensil placed on the cooking zone, the method from DE 10 2004 002 058 B3 proposes that the first heat sensor unit measure substantially a heat flow emanating downward only from the cooktop panel in the area of the cooking zone, and that the second and third heat sensor units measure substantially a heat flow emanating downward, in the area of the cooking zone, from the cooktop panel and a cooking utensil placed thereon. The heat sensor units are located beneath the lower surface of the cooktop panel in the region of a limited measuring spot which is delimited from the surroundings by a measurement channel or hollow waveguide, for example. The temperature Tcooking utensil of the cooking utensil bottom is determined based on the output signals of the heat sensor units and evaluated for purposes of controlling the heat output of the heating device in the electrical control system provided with the processing unit and the memory.
According to this method, it is not absolutely necessary to know the emissivity εcooking utensil of the bottom of the cooking utensil. However, taking the emissivity of the bottom of the cooking utensil into account would result in a more accurate measurement result of the temperature Tcooking utensil of the cooking utensil bottom. Therefore, German Patent DE 10 2004 002 058 B3 describes determining εcooking utensil by performing a reflection measurement. This method results in more accurate measurement results for the determination of the temperature of the cooking utensil bottom if the transmission coefficient of the cooktop panel is known and remains constant. However, during normal use of the cooktop panel, the transmission coefficient of the cooktop panel may vary because of accumulation of dirt. A dirty cooktop panel has reduced transmission, which leads to errors in the determination of εcooking utensil. In addition, this corrupts the results obtained in the measurement of the temperature of the cooking utensil bottom.